This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We recently measured the mean and variance of end-to-end length for a series of DNA double helices in solution, using small-angle x-ray scattering interference between gold nanocrystal labels. The observed variance in end-to-end length increases dramatically as a function of length. We have attributed this increase in variance to a soft stretching mode. Two technical comments have been submitted to Science;the authors suggest that the increase in observed variance as a function of length can be attributed to bending of the DNA. One way to test this hypothesis is to change the persistence length by modulating ionic strength and valence. If bending can explain the observed trend, measurements at either low ionic strength (10 mM Na+) or with a multivalent salt (putrescine2+) will decrease the persistence length and increase the variance. In order to prepare our response to the technical comment(s), we must make these measurements as soon as possible and establish the contribution of bending to our observed variances.